The present invention relates to disk drives and more particularly to an improved closed loop head positioning servo system for use in such drives.
Disk drives normally employ coarse positioning subsystems for moving heads to the proximity of addressed tracks on recording surfaces and fine positioning subsystems for moving those heads into final alignment with the addressed tracks. There are two basic categories of head positioning servo systems, open loop systems and closed loop systems.
In open loop head positioning servo systems, both coarse and fine positioning are controlled by indexing mechanisms that are mechanically or electrically coupled to the carriage assembly for the heads.
In one type of open loop system, a reticle is attached to the movable carriage assembly and a photocell is attached to a stationary support adjacent the carriage assembly. Movement of the carriage assembly causes the photocell to generate a modified sine wave. The peaks of the sine wave are related to the tracks crossed on the recording surface during movement of the carriage assembly. Pulses derived from the sine wave are used to decrement a register initially set at a count representing the number of tracks between the initial position of the carriage assembly and its desired position. When the register is decremented to zero, it is assumed the heads, which are rigidly attached to the carriage assembly, are aligned with the addressed track.
In some prior art head positioning servo systems, no further fine positioning is provided. The assumption is made that the head is positioned acceptably close to the center line of the desired track by the coarse positioning subsystem. In other prior art servo systems, a provided fine positioning subsystem is activated when the head approaches the addressed track. The fine positioning subsystem is sensitive to smaller increments of movement than the coarse positioning subsystems and therefore can be used to more precisely position the carriage assembly. Mechanical and magnetic equivalents of the above-described open loop servo systems are known in the prior art.
The disadvantage of all open loop servo systems is that such systems actually position the carriage assembly (and heads) with reference to a stationary part of the disk drive but not with reference to recorded data on the recording surfaces. If the heads in the carriage assembly are not in designed positions relative to the indexing mechanism on the carriage assembly, which can happen as a result of manufacturing process tolerances or abuse during shipment, installation or use of the disk drive, the indexing mechanism can position the carriage assembly exactly where it is supposed to be but the heads will remain offset from the center of the data tracks.
Even if the heads are located exactly as designed relative to the indexing mechanism, heads positioned by open loop systems may become offset from track center periodically during normal read/write operations. This is because a disk pack rotates about an axis which only theoretically remains vertical through the center of a drive spindle. In fact, disk packs normally wobble or revolve in an eccentric orbit about the spindle. As a result, an open loop system head, which becomes immovable once the carriage assembly reaches the desired position, may be centered on an addressed track only during a part of each revolution of the disk pack.
Open loop head positioning servo systems are satisfactory for disk drives in which the number of tracks per inch (track density) is low. Relatively large inter-track spacing and relatively wide recording tracks employed in such drives allow the open loop servo system to position the heads acceptably near, if not on, the center lines of the tracks.
For disk drives which read or write at higher track densities, the heads are generally positioned with reference to information written on the recording surfaces rather than with reference to a stationary support in the disk drive. Such systems are referred to as closed loop or track following servo systems.
In one widely used closed loop head positioning servo system, one surface of a disk is dedicated to tracks of pre-recorded servo information. A read only transducer or servo head detects the pre-recorded servo information on a selected servo track to provide signals which are processed by logic circuits. These signals are employed to cause the servo head and all other heads in a mechanically ganged head assembly to follow the selected servo track. A disadvantage of this type of closed loop system is that the servo head and the data heads may not be aligned with one another due to manufacturing process tolerances or abuse of the head assembly. Thus, notwithstanding the servo head may follow the centerline of the servo track in a selected cylinder, the data heads may be permanently offset by differing amounts from the centerlines of the data tracks in the same cylinder.
In another type of closed loop head positioning servo system, which could be described as a single surface system, servo information is recorded on the same surfaces on which data is to be recorded. In one such system, the servo information is recorded in a number of servo tracks separated from one another by one or more data tracks. Where the servo tracks are separated by a single data track, the servo information in adjacent servo tracks is normally recorded at differing frequencies far below the frequency at which the data is recorded. A single head can be used to read both data and servo information in a single channel. The term "channel" as used herein refers to that area on a disk surface which can be read by a single head without repositioning. In the system just described, a channel would consist of a data track and the two parallel servo tracks on opposite edges of the data track. By passing the detected signal through low frequency filters, the servo signals being read from adjacent servo tracks can be separated and processed to derive head position error signals. The head is driven to a position in which the two servo signals are equally strong; i.e., midway between the servo tracks and thus centered on the data track. The servo information recording technique described above is intended as an example only. A number of different techniques for recording and extracting servo information are known in the art.
In a variant of the single surface system described immediately above, the pre-recorded servo information is confined to a limited number of radial sectors on the disk surface. The sectors of the disk surface between the servo sectors are used for data. A single transducer is used to read and write data and also to detect the pre-recorded servo information as the transducer traverses a servo sector.
One disadvantage of the single surface systems relative to the dedicated servo surface system described earlier is that the single surface systems are subject to catastrophic write failures. If a head erroneously erases data from the recording surface, the servo information on the surface can be obliterated resulting in a partial or perhaps total loss of servo capability. The dedicated servo surface system is not subject to catastrophic write failure since its servo head is a read-only transducer.
Another disadvantage of the single surface systems is that a significant portion of the data surface is normally dedicated to the recording of servo information, reducing the data storage capacity of the surface.